1. Field of the Invention
This invention relates to a tuning circuit for AM receiver, wherein an antenna tuning circuit, a radio frequency amplifier circuit, and a local oscillator circuit are provided in the form of blocks each having a simplified construction using no trimmer capacitor, thereby facilitating non-adjustment of the AM receiver.
2. Description of the Prior Art
Generally, in an AM receiver of the superheterodyne type, a desired receiving frequency is selected at an antenna tuning circuit by adjusting a voltage applied to a variable-capacitance diode; the selected frequency signal is amplified in a semiconductor integrated circuit; and then the thus amplified signal is passed to a mixer circuit through a radio frequency amplifier circuit. Meanwhile, an oscillation frequency to be derived from a local oscillator circuit is automatically set by a PLL synthesizer; the oscillation frequency signal, along with the received frequency signal, is passed to the mixer circuit so that the received frequency signal is subjected to frequency-conversion; and a signal resulting from the frequency-conversion is passed to an intermediate frequency amplifier circuit.
In the conventional tuning circuit for AM receiver, an antenna tuning circuit, a radio frequency amplifier circuit, and a local oscillator circuit are individually provided on a printed circuit board. With such an arrangement, influences of stray capacitance and stray inductance are imparted to each stage through wiring patterns formed on the printed circuit board, so that the tuned frequency and/or oscillation frequency tends to be deviated from the theoretical design value. To eliminate such influences, it has been the practice to provide, at each stage, a trimmer capacitor and/or high frequency coil including a core for inductance adjustment; and by using such components for adjustment, tracking adjustment is performed to achieve a maximum receiving sensitivity in each receiving band. Obviously, however, the foregoing conventional arrangement is disadvantageous in that the number of components is increased and the number of assembling steps is also increased because of tracking adjustment.
Another disadvantage of the above-described conventional arrangement is such that because of the high frequency coils used in the tuning circuit being adjustable, it may happen, in use, that the cores of the coils are accidentally displaced due to vibration or the like so that a detuned state is likely to occur. Troublesomeness is also experienced in that a treatment such as paint locking of the inductance adjusting cores is usually required.
With the foregoing conventional arrangement, it is required that tracking adjustment be performed after all the components of the antenna tuning circuit, local oscillator circuit, intermediate frequency amplifier stage, low frequency amplifier stage and so forth have been mounted onto the printed circuit board. A further disadvantage is such that if a fault is detected as the consequence of the adjustment, then all of the components mounted on the printed circuit board must be discarded. Alternatively, the faulty component must be replaced with a new one, and then adjustment must be performed again. In either case, it is not economical.